Please, Explain me Listener Orientation in OpenAL, Why it set two vectors? And how it define?

the two vectors are the "looking at" vector, and the "up" vector. I think they needed a "right ear is pointing" vector so they could take care of panning left and right, and a "head is facing forward" vector so they could pan front to back, or perform neat HRTF stuff on it (no 3rd vector needed, since humans have lousy spatial perception in the up-down dimension anyway). However in most simple 2-d stuff, it would be simpler just to give the look-at vector, and the up vector (because up would stay constant), and it could do a cross-product to compute the right-ear vector. This would save the programmer some effort.

(Of course, in the case of full 3-d, the programmer can often extract the right-ear vector easily from the orientation matrix)

link

Actually, with a "looking at" vector, and an "up" vector, you can define one's orientation completly. This is why you only need two (and not three) vectors.

But you are correct. The orientation vectors are required to give the sources around you a direction. Otherwise you would have to convert to OpenAL's co-ordinate system and transform your source positions manually. From there it calculates how much each speaker should play the given sound.